1. Field of the Invention
The present invention relates to an inspection apparatus and a method for the inspection of a target on a substrate, for example, a surface of a substrate being processed in the semiconductor industry.
2. Description of the Related Art
A lithographic projection apparatus is used to image a pattern (e.g., in a mask) onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, and a hard bake. These procedures are used as a basis to pattern an individual layer of a device, e.g., an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Each procedure or process may be followed by an inspection of the substrate in an inspection apparatus. With the inspection results, one may optimize or improve the procedures prior to the inspection or if a large part of the substrate is faulty, the patterned layer may be stripped off the substrate and the stripped patterned layer may be reapplied through exposure and/or other lithographic processing. Further information regarding lithographic processes used in, for example, the semiconductor industry can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
The inspection apparatus may be used for surface inspection in semiconductor processing and may measure properties like line width, pitch, and critical dimension (CD) of the patterned layer. The apparatus may also measure the relative placement of one layer to another layer (i.e., overlay) and the layer thickness of a resist layer. An example of such an inspection apparatus, named MOXIE is described in “A new approach to Pattern Metrology” by Cristopher P. Ausschnitt, proceedings of SPIE, Vol. 5375, page 51 to 65, incorporated herein by reference.
The MOXIE sensor detects at least one non-zero diffracted order from a grating target in a “diffraction” channel and the reflected energy from both patterned and un-patterned illuminated areas in a “reflection” channel. The sensor is configured such that the illumination in the 300-700 nm range is directed along the direction of the grating period of the target at an angle relative to the Z-axis (the Z-axis being perpendicular to the target). The non-zero diffraction order rays are directed substantially in the Z-direction and a cylindrical lens in the “diffraction” channel projects the first order diffracted rays onto a detector array in a direction along the direction of the grating period. The detector array will measure the intensity of the non-zero diffraction order as a function of the wavelength.